The heat is stuck on in my car, which makes me not want to drive it in the summer. It’s an ‘87 V12 with the Mk3 system.

I have the temperature set all the way down, the fan off, and the knob pulled out for manual control, and yet warm air is coming from the center vents. What should I do to address this?

The air conditioning doesn’t work, which is a separate but maybe related problem. All the wiring to the compressor is missing. I intend to fix that, but in the meantime it would be nice if the system wasn’t adding heat to the cabin.

 

The heater valve might be stuck open....or perhaps is being commanded to stay open.

The valve has an external actuating rod/linkage. See if you can operate it by hand. It might be seized.

If linkage moves OK then apply vacuum to the valve. It should close. If not, the vacuum diaphragm is kaput.

If the valve itself is operating OK but stays open when the system is in cooling mode then we'll have to dig into the brains and controls of the system.

Or, as a stop gap, run a vacuum hose from the heater valve to one of the vacuum taps at the rear of the engine so that vacuum is always applied and the valve is always closed.

Cheers
DD

 

Thanks Doug! I replaced the valve a couple of years ago when I was going through the cooling system. I suppose it could be faulty anyway, I’ll test it when I get home. It’s also very possible the vacuum line has a hole in it somewhere, that is original to the car.

 

If you replaced the valve with a metal one, and you have vacuum engine running at the valve, get one of the plastic generic valves. Kirby Palm's BOOK recommends the one to get. Rock Auto sells them under the XJS listings.

Jon

 

Yes it is one of the metal valves. I didn’t realize the plastic ones were better. They’re pretty cheap too, so I’ll try going that route.

 

I have ordered a new valve. It will be here on Tuesday as I didn’t see the point in paying $30 for overnight shipping on a $7 part. In the meantime I’ll connect the current valve straight to the engine vacuum and see what happens.

 

Don’t be surprised if it’s a stuck blend flap, or stripped gears in blend flap motor, or blend flap is just confused about where it is. I solved my heat-on-always problem by taking the motor out, moving the flap by hand, finding nothing wrong, and putting it back together. Just that jostling alone was enough to wake it up and have it close the heat off when the knob is set to something like cold. I also found it was more likely the close the flap if I moved it all the way to hot, then moved it to cold maybe 25%, pause, then another 25%. If I went straight hot to cold it didn’t realized I’d turned the knob.

 

Can you get to the components you mentioned without pulling the whole unit out?

 

Yes, but you will have to pull the gauge cluster and glovebox liner to access the motor or the sensor on the top blend flap. There’s a motor on one side, and a position sensor on the other. The bottom pair are just behind the cheek panels and less of an ordeal to reach. The blend flap itself can be rotated with your fingers once the motor has been removed. You will want to move it with your fingers to make sure it’s not stuck. I think mine might have been jammed with dirt, leaves, old foam, whatever and exercising this blend flap seems to have solved the issue for me. When I say motor, it’s just a small plastic box with small plastic gears inside. You’ll want to peek inside and make sure the gears still have teeth. There’s plenty of used motors on ebay. If you see stripped teeth, be happy! It means you found the problem!

Just for giggles…. I have the factory dealership AC test equipment! I attempted to use it just to see how it would go. It never worked. Using a 35 year old computer to test a 35 year old computer is the height of frustration. The video is still interesting because it shows you how the PDU, the VBA, the VIA, and the AC computer are connected in all its late ‘80s glory.

 

Will be replacing the metal valve on our '94 in the next couple of weeks - I have vacuum to the valve, but it's not closing. Curious how much coolant needs to be drained and how you connect the vacuum line to the much larger connection on the plastic valve.

 

John,

I can't speak for a plastic replacement, but when I changed the metal valve on my 4 litre, I was able to do it losing an absolutely negligeable amount of coolant.

Paul

 

Useful data point, thank you!

 

I haven't replaced the valve yet, but I did run a vacuum line from the engine to the valve and that has closed the valve. Might get to the valve replacement on Saturday. As for the vacuum line fitting over the larger fitting on the plastic valve, I don't think it will be a problem if I replace the line. The existing line is very hard and about 3mm ID. The line I ran to the engine is new silicone 3mm ID and I had to stretch it a little to fit on the engine fitting I chose. Now I need to fine where the vacuum line to the heater goes...